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Tropical Storm ISAAC (Text)

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TROPICAL STORM ISAAC DISCUSSION NUMBER   7
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL092006
500 AM AST FRI SEP 29 2006
 
ISAAC IS SENDING MIXED STRUCTURAL SIGNALS THIS MORNING.  THE
CONVECTION HAS BECOME MORE CONSOLIDATED AROUND THE CENTER...
SUGGESTING THAT THE VERTICAL SHEAR IS DECREASING.  ON THE OTHER
HAND...THAT CONVECTION IS VERY WEAK...WITH ONLY A FEW TOPS COLDER
-50C.  THIS SUGGESTS THAT THE ONGOING INTERACTION WITH A NEARBY
UPPER-LEVEL LOW HAS HAD AN IMPACT...AND THAT ISAAC MAY HAVE SOME
SUBTROPICAL CHARACTERISTICS.  SATELLITE INTENSITY ESTIMATES REMAIN
35 KT FROM ALL AGENCIES...ALTHOUGH THE DVORAK DATA-T NUMBERS REMAIN
LESS THAN THAT.  THE INITIAL INTENSITY REMAINS 40 KT...AND THIS
COULD BE A LITTLE GENEROUS.

ISAAC HAS TURNED TOWARD THE LEFT OVER THE PAST 6-12 HR.  THE INITIAL
MOTION IS A RATHER UNCERTAIN 305/4...AND OVER THE PAST 3-6 HR THE
CYCLONE SEEMS TO BE MOVING ALMOST DUE WEST.  WATER VAPOR IMAGERY
INDICATES A LARGE MID/UPPER-LEVEL RIDGE BUILDING NORTH OF ISAAC IN
RESPONSE TO AN UPSTREAM DEEP-LAYER TROUGH OVER THE EASTERN UNITED
STATES.  THIS RIDGE IS LIKELY RESPONSIBLE FOR THE LEFT TURN.  THE
U. S. TROUGH IS FORECAST TO MOVE EASTWARD...WITH AN ASSOCIATED
SURFACE LOW FORMING OFF THE U. S. EAST COAST IN 48-72 HR.  THIS
DEVELOPMENT SHOULD TURN ISAAC NORTHWARD.  AFTER 72 HR...THERE IS
SOME MODEL DIVERGENCE.  THE UKMET...NOGAPS...AND CANADIAN MOVE
ISAAC NORTHEASTWARD IN ADVANCE OF THE NEW LOW...WHILE THE GFS AND
ECMWF MERGE ISSAC WITH THE NEW LOW AND BRING THE CENTER NEAR
EASTERN NOVA SCOTIA IN ABOUT 96 HR.  FOR NOW...THE NEW FORECAST
TRACK WILL RETAIN THE NORTHEASTWARD MOTION AFTER 72 HR SEEN IN THE
PREVIOUS ADVISORY.  THE NEW TRACK IS SHIFTED TO THE LEFT OF THE
PREVIOUS FORECAST...AND AN ADDITIONAL SHIFT TO THE LEFT MAY BE
NECESSARY LATER TODAY IF THE CURRENT WESTWARD TURN CONTINUES.

THE INTENSITY FORECAST IS SOMEWHAT PROBLEMATIC.  ISSAC SHOULD BE IN
A LIGHT TO MODERATE VERTICAL SHEAR ENVIRONMENT FOR 36-48 HR...AND
THE NEW FORECAST TRACK TAKES THE SYSTEM OVER SOMEWHAT WARMER SEA
SURFACE TEMPERATURES THAN SEEN EARLIER.  NEGATIVE FACTORS INCLUDE
THE CURRENT WEAK CONVECTION AND DRY AIR SEEN IN WATER VAPOR IMAGERY
NEAR AND SOUTH OF THE CYCLONE.  THE GFDL RESPONDS TO THESE FACTORS
BY FORECAST ISAAC TO BECOME A HURRICANE IN 72 HR.  THE FSU
SUPERENSEMBLE FORECASTS IT TO 48 KT IN 48 HR AND 61 KT IN 84
HR...WHILE THE SHIPS MODEL FORECASTS 44 KT IN 48 HR AND 55 KT IN 96
HR.  GIVEN THE CURRENT CONVECTIVE TRENDS...THE NEW INTENSITY
FORECAST WILL CHANGE LITTLE FROM THE PREVIOUS ADVISORY...CALLING
FOR GRADUAL STRENGTHENING DURING THE NEXT 36 HR.  AFTER THAT...
INTERACTION WITH THE APPROACHING DEEP-LAYER TROUGH SHOULD STOP
INTENSIFICATION AND LEAD TO EVENTUAL EXTRATROPICAL TRANSITION.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      29/0900Z 29.2N  55.6W    40 KT
 12HR VT     29/1800Z 29.7N  56.4W    40 KT
 24HR VT     30/0600Z 30.5N  57.7W    45 KT
 36HR VT     30/1800Z 31.5N  59.0W    50 KT
 48HR VT     01/0600Z 33.2N  60.0W    50 KT
 72HR VT     02/0600Z 39.5N  60.0W    50 KT...BECOMING EXTRATROPICAL
 96HR VT     03/0600Z 46.5N  51.0W    50 KT...EXTRATROPICAL
120HR VT     04/0600Z 49.0N  40.0W    50 KT...EXTRATROPICAL
 
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FORECASTER BEVEN
 
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